Two issues with mounting the fuel injection under the intake:
1) If there was a fuel leak it would go into the
oil, likely destroying the engine (might save a fire that destroys the car, on the other hand). I do not know if the
oil would attack the O-rings that seal the injector in the bungs.
2) The injector bungs should aim the injector at the intake valve. Cannot do this from underneath. At light loads the injector sprays fuel onto the intake valve before it opens. The hot valve boils the fuel, so that all the liquid changes phase into a gas. This give more complete burning of the fuel for better MPG and cleaner emissions.
I do believe that with a some ingenuity, the injectors and fuel rails could be inside the intake manifold. Inside is not necessarily in the air-stream, rather machined into the aluminum. This would require the design to be in pieces, such that you would disassemble it to get to the injectors and fuel rails.